1. Field of the Invention
The present invention relates to a wavelength conversion element used in wavelength division multiplexing optical communication or the like, and a method for using such a wavelength conversion element.
2. Description of Related Art
A variety of means for constructing a large-capacity optical communication network with a transmission speed of 1 Tbit/s or higher have been studied, and among them the wavelength division multiplexing (WDM) technology has attracted most attention. Wavelength conversion devices are necessary for realizing the WDM optical communication network.
For example, if a wavelength conversion device is employed in an optical cross-connect node, then collisions between channels are avoided and wavelength can be reused. Other advantages include easy network control and network modification (upgrade), which makes it possible to use new communication bands.
Quasi-phase matching (QPM) wavelength conversion elements (sometimes referred to hereinbelow as “QPM-type wavelength conversion elements”) which are the elements using an optical waveguide and in which wavelength conversion is carried out by realizing the QPM with a periodic domain inversion structure formed in the optical waveguide are used as elements constituting the aforementioned wavelength conversion devices. The optical waveguide is used because light propagating in the optical waveguide can propagate through a necessary distance, while maintaining a high energy density thereof. An optical waveguide formed in a QPM-type wavelength conversion element will be sometimes referred to hereinbelow as a QPM optical waveguide.
The periodic domain inversion structure is a structure composed by periodically arranging a plurality of domains with reversal spontaneous polarization of ferroelectrics, as described hereinbelow. This is why it is also called a periodic polarization reversal structure.
Here, for convenience of explanation, a light serving as a carrier wave which carries a signal that has to be transmitted in optical communication will be called a signal light, a light used for nonlinearly interacting with the signal light and converting the wavelength of the signal light will be called a pump light, and a signal light with the converted wavelength will be called a converted light.
The higher is the energy density of nonlinearly interacting lights (for example, a signal light and a pump light) and the longer is the length of nonlinear interaction (interaction length) the stronger are the nonlinear optical effects. Thus, producing a structure in which a nonlinear optical effect is realized in a waveguide in which light can propagate through the necessary distance, while maintaining a high energy density thereof, is effective for increasing the intensity of nonlinear optical effects.
Further, it is also effective to design the nonlinearly interacting lights (for example, a signal light and a pump light) so that they together can be coupled into the QPM waveguide efficiently, and propagate in an optical waveguide in a lowest order mode (fundamental mode). For this purpose, it is desired that the wavelengths of the nonlinearly interacting lights be almost equal. This is because that the optical coupling conditions depend on the wavelength of the light to be coupled into the waveguide, and a light with too shorter wavelength may not to be guided in a lowest order mode if the waveguide is designed as a single mode waveguide for a light with longer wavelength.
An example of the conventional wavelength conversion element of the above-described type is described below.
A method was suggested (see, for example, C. Q. Xu, et al. Appl. Phys. Let. Vol.63, p.3559 (1993)) for converting the wavelength of a signal light by difference frequency generation (DFG) using a QPM-wavelength conversion element in which a periodic domain inversion structure is formed in an optical waveguide. Thus, difference frequency generation of a plump light and signal light is induced, and the differential frequency light obtained thereby (sometimes referred to hereinbelow as “DF light”) becomes a converted light. The wavelength of the pump light used in such wavelength conversion is about half of the wavelength of the signal light or the wavelength of the converted light.
Further, a method for conducting wavelength conversion of a signal light by using a QPM-type wavelength conversion element in which a periodic domain inversion structure formed in an optical waveguide and realizing secondary nonlinear optical effects in a cascade manner has also been reported (see, for example, M. H. Chou, et al. IEEE Photonic Tech. Lett. Vol.11, p.653 (1999)). Thus, with this method, a pump light is used as a basic wavelength light and this light is wavelength converted into a second harmonic light (sometimes referred to hereinbelow as “SH light”) with a wavelength half that of the basic light by second harmonic generation (SHG). Further, there is a method for converting a signal light into a converted light (DF light) having a new wavelength by simultaneous DFG of the second harmonic light with a half wavelength and the signal light. In this wavelength conversion, the wavelength of the pump light is almost equal to the wavelength of the signal light and the wavelength of the converted light (DF light).
In order to explain the problems which are to be resolved by the present invention, first, the structure of the QPM-type wavelength conversion element and the operation principle thereof will be described with reference to the conventional wavelength conversion technology.
A method for converting the wavelength of a signal light by the DFG will be described below with reference to FIG. 1. FIG. 1 is a schematic model employed for explaining a conventional wavelength conversion device 32 composed by using a wavelength conversion element 10, a multiplexer 12, and a narrow-band wavelength filter 14. In this model hatching is used in respective places to display the presence of an optical waveguide or domain. Therefore, this hatching does not mean the cross-sectional shape of a three-dimensional structure. Further, similarly to FIG. 1, in FIG. 3 which will be referred to in subsequent explanation, hatching is employed for the same purpose, and this hatching, too, does not mean the cross-sectional shape of a three-dimensional structure.
The wavelength conversion element 10 used for such wavelength conversion is a QPM-type wavelength conversion element in which a periodic domain inversion structure 20 was created in the optical waveguide 22. The mechanism of wavelength conversion involves conducting the DFG of a pump light (wavelength λp) and a signal light (wavelength λs), and employing the DF light thus obtained as a converted light (λc).
Prior to explaining the operation principle of the wavelength conversion element 10, a method for forming the wavelength conversion element 10 will be explained. A method for forming the wavelength conversion element 10 that will be explained hereinbelow is applicable not only to the wavelength conversion element 10 in which the wavelength of the signal light is converted by the DFG, but also to the formation of a similar wavelength conversion element based on the QPM.
<Method for Forming Wavelength Conversion Element>
The domain reversal structure 20 is composed of a first domain 16 and a second domain 18. In the first domain 16 and second domain 18, the directions of spontaneous polarization of ferroelectric crystals which are the base materials constituting the wavelength conversion element 10 are at an angle of 180° with respect to each other. For example, z-cut LiNbO3 substrates can be used as the base materials constituting the wavelength conversion element. In the explanation provided hereinbelow, z-cut LiNbO3 substrates will be considered as ferroelectric crystal substrates, unless stated otherwise.
Such z-cut LiNbO3 substrates are single domain crystal structures in which the directions of spontaneous polarization are so arranged as to be perpendicular to the surface. The plane at the far end side of the spontaneous polarization vector is sometimes called a +z plane, and that at the base end side of the spontaneous polarization vector is sometimes called a −z plane.
The domain reversal region (second domain) 18 is formed by reversing the domain periodically in the +z plane of the LiNbO3 substrate. Therefore, the periodic domain inversion region 20 is composed of the domain (first domain 16) where the spontaneous polarization of the single domain crystal substrate is maintained and the domain (second domain 18) where the direction of spontaneous polarization was reversed. Thus, the direction of spontaneous polarization of the first domain 16 is the direction from the −z plane to the +z plane, whereas the direction of spontaneous polarization of the second domain 18 is from the +z plane to the −z plane.
The period of the periodic structure formed by the first domain 16 and second domain 18 is Λ. The wavelength conversion efficiency can be maximized by making the size d1 of the first domain 16 equal to the size d2 of the second domain 18. Thus, it is preferred that d1=d2 and Λ=d1+d2.
It is well known that in order to form the region with reversed direction of spontaneous polarization, either Ti is high-temperature thermally diffused into the region where the direction of spontaneous polarization of the z-cut LiNbO3 substrate is to be reversed or a high voltage is applied to this region. In order to conduct high-temperature thermal diffusion of Ti, a Ti thin film may be formed to a thickness of 50 nm by a vacuum deposition method or the like on the portion where the second domain 18 will be formed and then thermal diffusion may be conducted for 10 hours at a temperature of 1000° C. In order to reverse the direction of spontaneous polarization by applying a high voltage, an electrode may formed on the portion where the domain 18 is to be formed and a high voltage may be applied instantaneously.
Then, an optical waveguide 22 is so formed as to cross the periodic domain inversion structure formed in the z-cut LiNbO3 substrate. It is well-known that the optical waveguide can be formed by a H+—Li+ ion exchange method (also called a proton exchange method) in which benzoic acid serves as an exchange source. For example, an optical waveguide can be formed by coating the entire region with a metal mask, while leaving only the region where the optical waveguide 22 is to be formed, immersing for 2 hours in benzoic acid at a temperature of 200° C., removing the metal mask and benzoic acid, and conducting annealing for 6 hours in wet O2 atmosphere at a temperature of 350° C.
<Operation Principle of Wavelength Conversion Element>
The operation principle of a wavelength conversion element for converting the wavelength of a signal light by the DFG will be described with reference to FIG. 1. A pump light 26 with a wavelength λp of 0.77 μm and a signal light 28 with a wavelength λs of 1.55 μm are multiplexed in the multiplexer 12 and introduced as an incident light 29 into the QPM optical waveguide 22 of the wavelength conversion element 10. In the QPM optical waveguide 22, a DF light with a wavelength λc of 1.53 μm is generated by the DFG of the pump light 26 and signal light 28. Therefore, a light obtained by multiplexing the pump light with a wavelength λp of 0.77 μm, signal light 28 with a wavelength λs of 1.55 μm, and DF light with a wavelength λc of 1.53 μm are outputted as an outgoing light 30 from the QPM optical waveguide 22 of the wavelength conversion element 10.
The process of the above-described wavelength conversion will be described hereinbelow with reference to FIG. 2. In FIG. 2, a wavelength in micron units is plotted against the abscissa, and a light intensity is plotted in arbitrary units against the ordinate. The positions of the initial points of the arrows directed upward show wavelength center positions of the pump light (wavelength λp), signal light (wavelength λs), and DF light (converted light) with a wavelength λc, and the length of the arrows reflects the relative relationship by being proportional to respective light intensity.
The pump light 26 with a wavelength λp of 0.77 μm and the signal light 28 with a wavelength λs of 1.55 μm are shown in FIG. 2 by semicircular symbols and the broken line connecting them. This is a schematic representation of the relationship providing for the generation of the DF light with a wavelength λc of 1.53 μm by a nonlinear interaction such as DFG of the pump light 26 and signal light 28. This result can be interpreted as a conversion of the signal light 28 with a wavelength λs of 1.55 μm into a converted light with a wavelength λc of 1.53 μm through the pump light 26 with a wavelength λp of 0.77 μm.
The DF light which is a new light with a wavelength λc of 1.53 μm is generated by introducing the pump light 26 with a wavelength λp of 0.77 μm and the signal light 28 with a wavelength λs of 1.55 μm into the wavelength conversion element 10. Therefore, the outgoing light 30 from the wavelength conversion element 10 becomes a light obtained by multiplexing the pump light 26 with a wavelength λp of 0.77 μm, signal light 28 with a wavelength λs of 1.55 μm, and DF light with a λc of 1.53 μm.
The outgoing light 30 is filtered with a narrow-band wavelength filter 14 and only the DF light with a wavelength λc of 1.53 μm is taken out as the converted light 31. Thus, the signal light 28 with a wavelength λs of 1.55 μm is wavelength converted as the DF light with a wavelength λc of 1.53 μm and converted into the converted light 31 with the wavelength conversion device 32 comprising the multiplexer 12, wavelength conversion element 10, and narrow-band wavelength filter 14.
The wavelength λp of the pump light used in this wavelength conversion is 0.77 μm and the wavelength λs of the signal light is 1.55 μm. Therefore, the wavelength of the pump light is about half that of the signal light. However, as described hereinabove, it is very difficult to couple both the pump light and the signal light into the waveguide. For this reason, it is difficult to increase the efficiency of energy conversion from the signal light to the converted light. A method for conducting wavelength conversion of a signal light by inducing a cascade of secondary nonlinear optical effects was suggested. With this method, the wavelength of the pump light and the wavelength of the signal light can be made almost equal to each other.
A method for conducting wavelength conversion of the signal light by inducing the aforementioned secondary nonlinear optical effects in a cascade manner will be described below with reference to FIG. 3. With this method the pump light is considered as the base wave light and the wavelength thereof is converted into a second harmonic light (SH light) with a wavelength half that of the pump light by the SHG. At the same time, the signal light is converted into a converted light (DF light) having a new wavelength by the DFG of the SH light and signal light.
The pump light 56 with a wavelength λp of 1.54 μm and a signal light 58 with a wavelength λs of 1.55 μm are multiplexed in a multiplexer 42 and introduced as an incident light 59 into a QPM optical waveguide 52 of a wavelength conversion element 40. In the QPM optical waveguide 52, an SH light (wavelength λSH 0.77 μm) is generated by the SHG of the pump light 56 with a wavelength λp of 1.54 μm, and a DF light with a wavelength λc of 1.53 μm is generated by the DFG of the SH light and the signal light 58 with a wavelength λs of 1.55 μm.
Therefore, a DF light with a wavelength λc of 1.53 μm which is obtained by multiplexing the pump light with a wavelength λp of 1.54 μm, signal light with a wavelength λs of 1.55 μm, SH light with a wavelength λSH of 0.77 μm, and DF light with a wavelength λc of 1.53 μm is outputted as an outgoing light 60 from the QPM optical waveguide 52 of the wavelength conversion element 40.
The aforementioned wavelength conversion process will be described below with reference to FIG. 4. In this figure, a wavelength is plotted in the micron units against the abscissa shown in FIG. 4, similarly to FIG. 2, and the light intensity is plotted in arbitrary units against the ordinate. Furthermore, the starting point positions of the upward arrows show the wavelength center positions of the pump light (wavelength λp), signal light (wavelength λs), SH light (wavelength λSH), and DF light (converted light) with a wavelength of λc, and the length of the arrows reflects the relative relationship by being proportional to respective light intensity.
A second harmonic light with a wavelength λSH of 0.77 μm is generated by the secondary nonlinear optical effect from the pump light 56 with a wavelength λp of 1.54 μm. This is shown in FIG. 4 by drawing sidewise arrows from the arrows showing the pump light 56 with a wavelength λp of 1.54 μm to the arrow showing the SH light with a wavelength λSH of 0.77 μm.
The SH light with a wavelength λSH of 0.77 μm that was generated by the SHG and the signal light 58 with a wavelength λs of 1.55 μm are shown in FIG. 4 by semicircular symbols and the broken line connecting them. This is a schematic representation of the relationship providing for the generation of the DF light with a wavelength λc of 1.53 μm by a nonlinear interaction such as the DFG of the SH light and signal light 58. This result can be interpreted as a conversion of the signal light 58 with a wavelength λs of 1.55 μm into a converted light with a wavelength λc of 1.53 μm through the SH light and pump light 56 by realizing the SHG and DFG, which are nonlinear optical effects, in a cascade manner.
The DF light which is a new light with a wavelength λc of 1.53 μm is generated by introducing the pump light 56 with a wavelength λp of 1.54 μm and the signal light 58 with a wavelength λs of 1.55 μm into the wavelength conversion element 40. Therefore, the outgoing light 60 from the wavelength conversion element 40 becomes a light obtained by multiplexing the pump light 56 with a wavelength λp of 1.54 μm, signal light 58 with a wavelength λs of 1.55 μm, SH light with a wavelength λSH of 0.77 μm, and DF light with a λc of 1.53 μm.
The outgoing light 60 is filtered with a narrow-band wavelength filter 44 and only the DF light with a wavelength λc of 1.53 μm is taken out as the converted light 61. Thus, the signal light 58 with a wavelength λs of 1.55 μm is wavelength converted as the DF light with a wavelength λc of 1.53 μm and converted into the converted light 61 with the wavelength conversion device 62 comprising the multiplexer 42, wavelength conversion element 40, and narrow-band wavelength filter 44.
The wavelength λp of the pump light used in this wavelength conversion is 1.54 μm and the wavelength λs of the signal light is 1.55 μm. Therefore, the wavelength of the pump light is almost equal to the wavelength of the signal light. Therefore, an optical waveguide can be easily designed such that the pump light 56 with wavelength λp of 1.54 μm and the signal light 58 with a wavelength λs of 1.55 μm can be coupled into the waveguide easily and propagate in a lowest order mode.
However, as described hereinabove, because the wavelength λp of the pump light is 1.54 μm and the wavelength λs of the signal light is 1.55 μm, the position of the pump light with a wavelength λp of 1.54 μm on the wavelength axis is between those of the signal light (wavelength λs is 1.55 μm) and converted light (wavelength λc is 1.53 μm). For this reason, if this method is used for optical communications, the pump light occupies at least one communication wavelength channel.
Another problem is that, as has already been mentioned, central wavelengths of the signal light spectrum and pump light spectrum differ only by 0.01 μm (=10 nm), as FIG. 4 clearly shows. Therefore, base portions of the spectra overlap and the number of channels that can be essentially used for optical communication is limited.
Furthermore, if the power of pump light is several tens of mW, the wavelength conversion efficiency (energy conversion efficiency) by the secondary nonlinear optical effects is about several percents at maximum. Thus, a substantial wavelength conversion efficiency of conversion of the signal light into the DF light becomes about one tenth of a percent because it is a product of the secondary harmonic generation efficiency (several percents) and conversion efficiency to the DF light (also several percents). Therefore, if a method is used by which the secondary nonlinear optical effects are induced to conduct wavelength conversion of the signal light, then the conversion efficiency will be about one tenth of a percent and substantially lower than the wavelength conversion efficiency of the method by which the signal light is directly converted into the DF light.
Accordingly, it is a first object of the present invention to provide a wavelength conversion element capable of converting the wavelength of a signal light with a high wavelength conversion efficiency.
Further, it is a second object of the present invention to provide a wavelength conversion element such that the wavelength of a pump light can be set to a wavelength longer or shorter than the wavelength of a plurality of signal lights and a plurality of converted lights present in the optical communication band and the wavelength band of the pump light can be set separately from the wavelength band of the signal lights and converted lights.
It is a third object of the present invention to provide a method for using the above-described wavelength conversion element.